I blogged about this in 2011 in a post called A warm river runs through it. There are a couple of large rivers that discharge into the Arctic Ocean, such as the Ob, Lena and Yenisei in Russia, and the Mackenzie river in Canada. During spring and summer, these rivers carry warmth from lower latitudes up to the Arctic which helps melting coastal fast ice and sea ice. This impact is on the increase, as the globe continues to warm.
Warm Rivers Play Role in Arctic Sea Ice Melt
The heat from warm river waters draining into the Arctic Ocean is contributing to the melting of Arctic sea ice each summer, a new NASA study finds.
A research team led by Son Nghiem of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., used satellite data to measure the surface temperature of the waters discharging from a Canadian river into the icy Beaufort Sea during the summer of 2012. They observed a sudden influx of warm river waters into the sea that rapidly warmed the surface layers of the ocean, enhancing the melting of sea ice. A paper describing the study is now published online in the journal Geophysical Research Letters.
I distinctly remember discussions here on the ASIB, quite early in the 2012 melting season, with regards to the influence warm Mackenzie water possibly had on the early, massive retreat of sea ice in the Beaufort Sea.
The team said the impacts of these warm river waters are increasing due to three factors. First, the overall volume of water discharged from rivers into the Arctic Ocean has increased. Second, rivers are getting warmer as their watersheds (drainage basins) heat up. And third, Arctic sea ice cover is becoming thinner and more fragmented, making it more vulnerable to rapid melt. In addition, as river heating contributes to earlier and greater loss of the Arctic’s reflective sea ice cover in summer, the amount of solar heat absorbed into the ocean increases, causing even more sea ice to melt.
In turn, rivers probably heat up more quickly because of the increase in the rate of snow cover melt during spring and summer.
The team analyzed data from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on NASA’s Terra satellite to examine sea ice patterns and sea surface temperatures in the Beaufort Sea. They observed that on June 14, 2012, a stretch of landfast sea ice (sea ice that is stuck to the coastline) formed a barrier that held the river discharge close to its delta. After the river water broke through the ice barrier, sometime between June 14 and July 5, the team saw that the average surface temperature of the area of open water increased by 11.7 degrees Fahrenheit (6.5 degrees Celsius).
“When the Mackenzie River’s water is held back behind the sea ice barrier, it accumulates and gets warmer later in the summer,” said Nghiem. “So when it breaks through the barrier, it’s like a strong surge, unleashing warmer waters into the Arctic Ocean that are very effective at melting sea ice. Without this ice barrier, the warm river waters would trickle out little by little, and there would be more time for the heat to dissipate to the atmosphere and to the cooler, deeper ocean.”
An interesting, slightly counterintuitive theory. Fast ice holding the river back actually increases the effect the river has on sea ice retreat. If correct, one would assume that river discharge this year won't be as big as in 2012. Firstly, the Arctic was relatively warm for much of this winter, so maybe not enough fast ice has formed to create that barrier that results in a surge of warm water. Secondly, a lot of the cold was spread over North America and Siberia, so it might take a while longer for the rivers to heat up.
The team estimated the heating power carried by the discharge of the 72 rivers in North America, Europe and Asia that flow into the Arctic Ocean. Based on published research of their average annual river discharge, and assuming an average summer river water temperature of around 41 degrees Fahrenheit (5 degrees Celsius), they calculated that the rivers are carrying as much heat into the Arctic Ocean each year as all of the electric energy used by the state of California in 50 years at today’s consumption rate.
While MODIS can accurately measure sea surface temperature where rivers discharge warm waters into the Arctic Ocean, researchers currently lack reliable field measurements of subsurface temperatures across the mouths of river channels. Nghiem said more studies are needed to establish water temperature readings in Arctic-draining rivers to further understand their contribution to sea ice melt.
NASA monitors Earth’s vital signs from land, air and space with a fleet of satellites and ambitious airborne and ground-based observation campaigns. NASA develops new ways to observe and study Earth’s interconnected natural systems with long-term data records and computer analysis tools to better understand how our planet is changing. The agency shares this unique knowledge with the global community and works with institutions in the United States and around the world that contribute to understanding and protecting our home planet.
Read the entire press release here.